The present invention relates generally to (i) linear variable differential transformers (LVDTs), devices that convert very small mechanical displacements, as small as those in the sub-nanometer level, into differential voltages (and vice versa), and (ii) integrating LVDTs into the structure of a scanning probe device such as the atomic force microscope (AFM) so that certain movements of the device may be conveniently sensed and corrected if desired.
FIG. 1 shows an LVDT according to U.S. Pat. No. 7,038,443, Linear Variable Differential Transformers for High Precision Position Measurements, by some of the same inventors as here. This LVDT reflects the basic idea of these devices in the prior art that the mutual inductances between a moving primary and two secondaries change as a function of the position of a moving part. In commercial LVDTs available the LVDT of U.S. Pat. No. 7,038,443, the moving part was a ferromagnetic core and the positions of the primary and the secondaries was fixed. However, because of the use of non-ferromagnetic materials in its construction, the fact that the primary moves rather than being stationary and the advanced signal conditioning electronics controlling its operation, the LVDT of U.S. Pat. No. 7,038,443 provides sensitivity unavailable in previous LVDTs. The FIG. 1 LVDT comprises a movable non-ferromagnetic coil form 114 around which a primary coil 115 is wound and a stationary non-ferromagnetic coil form or forms 110 around which two secondary coils 103 and 104 are wound. The coil forms can be made of plastic or paramagnetic material. The primary coil form 114 is mechanically connected to the object of interest (not shown) by a shaft 108. The shaft 108 can transmit displacements of the object of interest on the order of microns or smaller. Alternatively the primary coil form could be stationary and the secondary coil forms could be movable with the object of interest mechanically connected to the secondary coil forms. The functionality of such a LVDT would be equivalent to that shown in FIG. 1.
Excitation electronics 111 produce the current driving the primary coil 115. As the position of the object of interest attached to shaft 108 changes, and therefore the position of the primary coil 115 with respect to the secondary coils 103 and 104 changes, the flux coupled to the two secondaries, 103 and 104, also changes. These voltages are amplified with a differential amplifier 106 and converted to a voltage proportional to the core displacement by the signal conditioning electronics 112. For small displacements, the signal is linear. The use of plastic or paramagnetic material in the construction of the FIG. 1 LVDT lowers the sensitivity gain that would be provided by high permeability magnetic material, but eliminates Barkhausen noise. The elimination of Barkhausen noise permits the output of the excitation electronics 111 to be raised without causing a corresponding increase in output noise, thus increasing the sensitivity of the LVDT.
FIG. 2 shows a more detailed depiction of the digital excitation and signal conditioning electronics for the FIG. 1 LVDT, taken from US Patent App. Pub. No. US20040056653, Linear Variable Differential Transformer with Digital Electronics, by some of the same inventors as here. The FIG. 2 digital excitation and signal conditioning electronics are based on a digitally generated square wave, which when filtered produces a sine wave drive signal with more precisely defined amplitude and frequency, and lower noise, than a sine wave drive signal generated by an analog sine wave generator. This digitally generated square wave originates in a microprocessor 280. The microprocessor could be a digital signal processor, a microcontroller or other similar microprocessors known to those skilled in the art. The square wave in turn is filtered by a low pass filter 224 that effectively removes all the harmonics of the square wave above the fundamental, resulting in a pure sine wave. The filter is optimized to be stable with respect to variations in temperature. The sine wave in turn is amplified by a current buffer 225 that directly drives the LVDT primary 215. A sine wave generated by this excitation circuit has nearly perfect frequency and amplitude stability and has a high signal to noise ratio.
In the embodiment of the excitation and signal conditioning electronics depicted in FIG. 2, one lead from each of the secondaries 103 and 104 is grounded and the other is connected to a high precision, low noise differential amplifier 106 which subtracts the input of one secondary from the input of the other and amplifies the difference mode signal. The differential amplifier is designed to produce low noise when coupled to a low impedance input source (such as a coil). The signal from the differential amplifier 106 is input to a buffer amplifier 231 and an inverting buffer amplifier 232. The output of the buffer amplifier 231 is fed into a normally closed input of an analog switch 233 while the output of the inverting buffer amplifier 232 is fed into a normally open input of the same switch. This arrangement could be reversed with no loss of functionality as long as the two inputs of the switch are set so that one input is open when the other input is closed. The action of the analog switch 233 is controlled by a square wave originating in the microprocessor 280 which can be phase shifted relative to the square wave also originating in the microprocessor 280 which (when filtered and amplified) drives the LVDT primary 215. Alternatively to a phase shift relative to the primary drive square wave originating in the microprocessor 280, it is possible to shift the phase relative to the signal going into the primary drive current buffer 225. All that matters is that the phase of the primary drive relative to the phase of the reference square wave is adjustable. Preferably, the opening of one input which occurs with the closing of the other input of switch 233 is 90 degrees out of phase with the output signal from amplifier 106. The output of the analog switch 233 is fed into a stable, low noise, low pass filter 234. The output of this filter provides a signal proportional to the position of the moving primary coil 215.
Scanning probe devices such as the atomic force microscope (AFM) can be used to obtain an image or other information indicative of the features of a wide range of materials with molecular and even atomic level resolution. As the demand for resolution has increased, requiring the measurement of decreasingly smaller forces and movements free of noise artifacts, the old generations of these devices are made obsolete. The preferable approach is a new device that addresses the central issue of measuring small forces and movements with minimal noise.
For the sake of convenience, the current description focuses on systems and techniques that may be realized in a particular embodiment of scanning probe devices, the atomic force microscope (AFM). Scanning probe devices include such instruments as AFMs, scanning tunneling microscopes (STMs), 3D molecular force probe instruments, high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, NanoIndenters, chemical or biological sensing probes, instruments for electrical measurements and micro-actuated devices. The systems and techniques described herein may be realized in such other scanning probe devices, as well as devices other than scanning probe devices which require precision, low noise displacement measurements.
An AFM is a device which obtains topographical information (and/or other sample characteristics) while scanning (e.g., rastering) a sharp tip on the end of a probe relative to the surface of the sample. The information and characteristics are obtained by detecting changes in the deflection or oscillation of the probe (e.g., by detecting small changes in amplitude, deflection, phase, frequency, etc.) and using feedback to return the system to a reference state. By scanning the tip relative to the sample, a “map” of the sample topography or other characteristics may be obtained.
Changes in the deflection or oscillation of the probe are typically detected by an optical lever arrangement whereby a light beam is directed onto the side of the probe opposite the tip. The beam reflected from the probe illuminates a position sensitive detector (PSD). As the deflection or oscillation of the probe changes, the position of the reflected spot on the PSD also changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the probe are typically made to trigger a change in the vertical position of the base of the probe relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the XY plane defined by the sample), in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that is typically used to generate an AFM image.
AFMs can be operated in a number of different sample characterization modes, including contact modes where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.
Actuators are commonly used in AFMs, for example to raster the probe over the sample surface or to change the position of the base of the probe relative to the sample surface. The purpose of actuators is to provide relative movement between different parts of the AFM; for example, between the probe and the sample. For different purposes and different results, it may be useful to actuate the sample or the probe or some combination of both. Sensors are also commonly used in AFMs. They are used to detect movement, position, or other attributes of various components of the AFM, including movement created by actuators.
For the purposes of this specification, unless otherwise indicated (i) the term “actuator” refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures; piezo tubes; piezo stacks, blocks, bimorphs and unimorphs; linear motors; electrostrictive actuators; electrostatic motors; capacitive motors; voice coil actuators; and magnetostrictive actuators, and (ii) the term “sensor” or “position sensor” refers to a device that converts a physical quantity such as displacement, velocity or acceleration into one or more signals such as an electrical signal, and vice versa, including optical deflection detectors (including those referred to above as a PSD), capacitive sensors, inductive sensors (including eddy current sensors), differential transformers (such as described in U.S. Pat. No. 7,038,443 and co-pending applications US Patent App. Pub. No. US20020175677, Linear Variable Differential Transformers for High Precision Position Measurements, and US20040056653, Linear Variable Differential Transformer with Digital Electronics, which are hereby incorporated by reference in their entirety), variable reluctance sensors, optical interferometry, strain gages, piezo sensors and magnetostrictive and electrostrictive sensors.